1. Field of the Invention
This invention relates to cranes and more particularly to lattice boom cranes having boom sections which telescope to increase boom length.
2. Prior Art
The boom sections in telescoping cranes are generally extended and retracted by hydraulic cylinders which act between the boom sections. These designs have not included pendant lines and the boom has necessarily been designed as a cantilevered beam. The additional strength required to withstand these bending loads, which are not present in a pendant line supported lattice boom, has dictated the use of heavy fabricated structural sections for the boom.
While the telescoping crane design is beneficial because of the versatility of providing a variable length boom, the prior art structures have required such massive boom structures to support the hydraulic cylinders and associated structure used to telescope the boom sections that the capabilities of the crane have been limited. While this additional boom weight has not severely affected the crane capability at near vertical boom angles, the capacity of the crane has been substantially affected at the lower, near horizontal boom angles.
Therefore, although the lattice boom structure is normally preferred in a fixed boom length crane, heretofore this structure has not been feasible for use in telescoping cranes. While the telescoping cranes have been produced with solid wall boom sections, the capability of the crane has been severely limited due to the excessive weight introduced by the boom structure. Therefore, a need has arisen for a new configuration of crane which would provide a variable length boom with the payload capability of a lattice structure.